The present invention generally relates to a method for cleaning wafers and more particularly, relates to a method for cleaning wafers with deionized water by first doping the water with ions to reduce the resistivity of the water such that electrostatic discharge problem does not occur on the wafer surface.
In the manufacture of semiconductor devices, a large quantity of deionized (DI) water is required to process integrated circuit wafers. The consumption of DI water increases with the size of the wafers. For instance, the consumption at least doubles in the processing of 200 mm size wafers when compared to the consumption in the processing of 150 mm size wafers. DI water is most frequently used in tanks and scrubbers for the frequent cleaning and rinsing of wafers in process. It is desirable that the surface of a wafer be cleaned by DI water after any process has been conducted on the wafer, i.e., oxide deposition, nitride deposition, SOG deposition or any other deposition or etching process. Such wafer cleaning step is accomplished by equipment that are installed either in-line or in a batch-type process.
For instance, a cassette-to-cassette wafer scrubbing system is one of the most used production systems for wafer cleaning prior to either a photoresist coating, oxidation, diffusion, metalization or CVD process. A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high pressure spray of DI water with a retractable cleaning brush. A typical mechanical scrubbing process consists of rotating a brush near a wafer surface that is sprayed with a jet of high pressure DI water at a pressure between about 2,000 and about 3,000 psi. The brush does not actually contact the wafer surface, instead, an aquaplane is formed across the wafer surface which transfers momentum to the DI water. The movement of the DI water thus displaces and dislodges contaminating particles that have been deposited on the wafer surface. Contaminating particles are thus removed by a momentum transfer process. As a result, larger particles become more difficult to dislodge and remove from a wafer surface.
A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. In a typical wafer scrubbing equipment, production rates are generally between 60 to 120 wafers per hour depending on the program length. The spinning speed of the wafer is between 500 to 10,000 rpm while under a water pressure of up to 6,000 psi.
In more recently developed wafer scrubbing systems, in-line systems are used which provide high pressure DI water scrubbing only while eliminating the possibility of wafer contamination by overloaded brushes. The water pressure in these systems range between 3,000 to 6,000 psi which are ejected from a nozzle mounted on an oscillating head. The wafer is spun when the oscillating spray is directed onto the wafer surface. After the cleaning step, wafer is dried by a pure nitrogen gas purge to promote rapid drying. After the scrubbing operation, wafers can be loaded into an in-line dehydration baking system for thorough drying. Batch-type systems are also used with DI water for cleaning, rinsing and drying prior to many IC processes. The systems can be programmed wherein wafers are loaded in cassettes before each cycle. One disadvantage of the batch system is their inability to be integrated into part of an automated wafer processing line.
In the conventional DI water cleaning systems, the basic requirements for the DI water cleaning system are that it provides a continuous supply of ultra-clean water with very low ionic content. It is believed that ionic contaminants in water, such as sodium, iron or copper when deposited onto a wafer surface can cause device degradation or failure. It is therefore desirable to eliminate all such ionic content from a DI water prior to using the water for cleaning wafers.
A conventional method of measuring the ionic content in DI water is by monitoring the water resistivity. A water resistivity of 18xc3x97106 Ohm-cm or higher indicates a low ionic content in the DI water. In a conventional water purifying system, several sections which include charcoal filters, electrodialysis units and a number of resin units to demineralize the water are used for purifying the water.
In the use of scrubbers to clean wafer surfaces, as scrubbers become a popular method to remove contaminating particles from wafer surfaces, a different problem is caused by the scrubbing action with DI water. When a DI water is extremely clean, i.e., without any ions present in the water, as indicated by a high resistivity of 18xc3x97106 Ohm-cm or higher, the DI water becomes a perfect insulating material. During a cleaning process of another perfect insulating material such as an oxide, nitride or SOG layer deposited on a wafer, the two insulating materials create an electrostatic charge on the wafer surface from the impingement action of the water jet on the wafer surface. The electrostatic charge build up on the surface of the wafer if not properly discharged, can result in severe damage to the devices already formed on the wafer. For instance, a common damage that is frequently observed is the shorting through of a gate oxide layer resulting in a device failure. The device failure then results in a serious drop in the product ion yield.
Referring initially to FIG. 1 wherein a graph illustrating a low yield wafer map obtained on a wafer sample that was processed by a conventional DI water scrubbing cleaning process is shown. In the graph, 0 indicates IC chips that are of a passing grade, while 3 indicates those IC chips that have failed and must be scrapped. It is seen that along the edges of the wafer map, most IC chips have failed due to the electrostatic discharge problems. The center portion of the wafer map indicates a majority of good chips were obtained since they were more shielded from the electrostatic discharge by the IC chips that surround them. The wafer map shown in FIG. 1 indicates a severely damaged wafer by the electrostatic discharge after a wafer cleaning process by non-doped DI water. Such a wafer would not be acceptable if a reasonable yield from the process is expected.
Referring now to FIG. 2, wherein the dependence of wafer yield on the electrostatic field strength is shown. It is seen that as the electrostatic field induced by static charge increases in the horizontal axis, the wafer yield is significantly affected and deteriorates from 93% to 91%. This represents a significant drop in the wafer yield and therefore a undesirable result due to damages caused by the stronger electrostatic field formed on the wafer surface.
Referring now to FIG. 3, wherein a graph illustrating the dependence of the electrostatic field formed on the DI water pressure is shown. The DI water pressure tested is in the range between about 1,200 psi and about 3,500 psi. It is seen that at approximately 1,200 psi water pressure, a 0.05 kV/inch of electrostatic field was formed. The electrostatic field strength greatly increases to 1.13 kV/inch and 1.16 kV/inch when the DI water pressure is increased to about 2,100 psi and about 2,900 psi, respectively. There is a large increase in the electrostatic field strength between 1,200 psi and 2,100 psi water pressure by almost 20 fold. The DI water pressure that impinges on the wafer surface is therefore a strong factor on the subsequent electrostatic field formed.
A study on the dependence of the electrostatic field strength on the nature of the coating material on the wafer surface is shown in FIG. 4. The data of the electrostatic field strength in kV/inch is used on the vertical axis, while different wafer sample lots are plotted on the horizontal axis. It is seen that for wafers that have a PREFERRED EMBODIMENT-oxide layer deposited on top and was subjected to DI water spray, the electrostatic field strength generated was between about 5 and about 6.2 V/inch. This is significantly higher, i.e., by approximately 5 fold of those generated on wafers that have a metal layer deposited on top. It is obvious that the metal layer when rubbing against the DI water does not generate any significant amount of electrostatic charge due to the conductive nature of metal.
It is therefore an object of the present invention to provide a method for cleaning a wafer surface with DI water that does not have the drawbacks and shortcomings of the conventional DI water scrubbing process.
It is another object of the present invention to provide a method for cleaning a wafer surface with DI water that does not produce an electrostatic field on the wafer surface.
It is a further object of the present invention to provide a method for cleaning a wafer surface with DI water that does not cause damages to the wafer due to electrostatic discharge problems.
It is another further object of the present invention to provide a method for cleaning a wafer surface in a scrubber utilizing DI water by first doping the water with suitable ions.
It is still another object of the present invention to provide a method for cleaning a wafer surface in a scrubber by DI water by doping the water with at least one species of ions selected from the group of OHxe2x88x92, Fxe2x88x92, H+, Clxe2x88x92 and NH4+ and H+.
It is yet another object of the present invention to provide a method for cleaning a wafer surface in a scrubber utilizing DI water by doping the water with ions such that its resistivity falls substantially below 18xc3x97106 Ohm-cm.
It is still another further object of the present invention to provide a method for cleaning a wafer surface in a scrubber by DI water in which the DI water is first doped with ions such that a significantly lower resistivity in the water, i.e., less than 104 Ohm-cm is obtained.
It is yet another further object of the present invention to provide a method for cleaning a wafer surface in a scrubber by using DI water in which a carboxylic acid or atomic acid is first mixed with the DI water to reduce its resistivity.
In accordance with the present invention, a method for cleaning a wafer surface in a scrubber by using deionized water is provided in which the DI water is first doped with at least one species of ions such that the resistivity of the water is significantly reduced so that electrostatic charge cannot be produced on the wafer surface during cleaning.
In a preferred embodiment, a method for cleaning a wafer surface in a scrubber can be carried out by first providing a quantity of deionized water, then adding ions to the quantity of deionized water such that the electrical resistivity of the water is reduced sufficiently so as to substantially not generating any electrostatic charge on the surface of the wafer, and then contacting the deionized water with a wafer surface.
In another preferred embodiment, a method for cleaning a wafer in a scrubber can be carried out by the steps of first providing a quantity of deionized water to a scrubber, then doping the quantity of deionized water with at least one species of ions such that the electrical resistivity of the water is less than 18xc3x97106 Ohm-cm, and then contacting a wafer with the doped deionized water.
In yet another preferred embodiment, a method for cleaning wafer surfaces in a scrubber by ion doped deionized water without electrostatic discharge problems can be carried out by first positioning at least one wafer in a scrubber, then providing a quantity of deionized water to the scrubber, then doping the quantity of deionized water with at least one species of ions selected from the group consisting of OHxe2x88x92, Fxe2x88x92, Clxe2x88x92, NH4+ and H+, and then contacting the ion doped water with the wafer surface.